From the Yonkou, we mostly know about Blackbeard's powers. He has a logia and a paramecia, the strongest one at that. From what we know, the Yami Yami no Mi can absorb devil fruit powers which somehow makes it unique.
Definition of the Yami Yami No Mi according to Wiki,
"The first and foremost strength, as demonstrated by Blackbeard, is that it allows the user to control darkness and its unique property of gravity. The darkness is visually demonstrated by a black smoke-like substance spreading out from the user's body. The darkness is a void that devours and crushes everything. Due to this, the user can absorb physical matter and attacks of any type by sucking it into the darkness. The user can also use this ability to irresistibly pull the opponent to them, regardless of their current state."
I think because of that unique ability, it is the strongest logia as it can counter other elements effectively. Attacks consisting of powerful elements like light is completely useless because darkness can absorb light.(Reference to black hole technique used by BB)
So, Blackbeard=Strongest Paramecia and Logia
Shanks=Strongest Haki (NOT CONFIRMED)
Kaidou= Strongest Zoan, possibly the emperor with the most physical strength and endurance. It is possible that he might be cyborg-like just like Franky but much stronger. (SPECULATION)
What about big mom?
Definition of the Yami Yami No Mi according to Wiki,
"The first and foremost strength, as demonstrated by Blackbeard, is that it allows the user to control darkness and its unique property of gravity. The darkness is visually demonstrated by a black smoke-like substance spreading out from the user's body. The darkness is a void that devours and crushes everything. Due to this, the user can absorb physical matter and attacks of any type by sucking it into the darkness. The user can also use this ability to irresistibly pull the opponent to them, regardless of their current state."
I think because of that unique ability, it is the strongest logia as it can counter other elements effectively. Attacks consisting of powerful elements like light is completely useless because darkness can absorb light.(Reference to black hole technique used by BB)
So, Blackbeard=Strongest Paramecia and Logia
Shanks=Strongest Haki (NOT CONFIRMED)
Kaidou= Strongest Zoan, possibly the emperor with the most physical strength and endurance. It is possible that he might be cyborg-like just like Franky but much stronger. (SPECULATION)
What about big mom?